The present invention relates to semiconductor fabrication, and more particularly to removing residue from a semiconductor wafer that has been subjected to planarization.
The present-day fabrication of semiconductor wafers is a complex multi-step process. In a typical process, various materials are sequentially applied to a substrate in order to build up a substantially laminar structure. After a layer has been laid down, selected regions of that layer are commonly modified or removed. Such manipulations often impart nonplanarity to the top surface of the layer. Nonplanarity, also known as uneven surface topography, is undesirable for a number of reasons, one of which is that laying down a subsequent layer is made more difficult if the underlying layer is nonplanar. Therefore, a common step during semiconductor wafer fabrication is to planarize the surface of the wafer, by a process known as planarization or polishing, where these terms are often used interchangeably.
There are a large number of specific processes that can be used to planarize the surface of a semiconductor wafer, a few of which will be discussed later herein. However, it is generally the case that planarization will selectively remove surface material that constitutes the highest points of the surface, i.e., the points of the surface furthest from the base of the semiconductor wafer. In this way, the high points are removed and the surface topography of the wafer is planarized, also called leveled or flattened. The planarization process can be carried out on an already planar surface, in which case an entire layer of a semiconductor wafer may be removed. Regardless of the amount of material that is removed by a planarization process, a typical consequence of planarization is that a residue is left on the planarized surface. This residue may be termed planarization residue.
The exact identity of the residue depends on the details of the planarization process, however at a minimum the residue typically includes bits and particles which have been dislodged from the surface, and often includes materials that were used to assist the planarization process. At the end of the planarization process, this residue should be completely removed before any further layers may be laid down upon the semiconductor surface.
Pure water, optionally in combination with scrubbing, is commonly used in the art to remove planarization residue. However, the prior art also describes methods and compositions which have been developed to more effectively and efficiently remove planarization residue from a semiconductor surface. For example, U.S. Pat. No. 5,478,436 to Winebarger et al. is directed to applying a cleaning solution to a semiconductor substrate having metal contaminants thereon, in order to remove the metal contaminants. The Winebarger et al. cleaning solution comprises an organic solvent and a compound containing fluorine. As another example, U.S. Pat. No. 5,389,194 to Rostoker et al. is directed to a method of cleaning polishing residue from a semiconductor device. The Rostoker et al. method uses a cleaning solution consisting essentially of phosphoric acid and hydrofluoric acid.
As recognized by both Winebarger et al. and Rostoker et al., a problem with the use of chemical-containing cleaning solutions for removing planarization residue is that, in order for the cleaning process to be effective, the cleaning solutions are so strong that they degrade/dissolve the surface layer itself. That is, the cleaning solutions cause undesirable degradation and/or removal of the surface of the planarized layer. While Winebarger et al. and Rostoker et al. both purport to address this problem, there is still a significant need in the art for satisfactory processes and cleaning solutions for selectively removing planarization residue from a semiconductor surface.
The present invention provides a process for effectively removing planarization residue from a planarized surface, with acceptable affects on the planarized surface itself. The process comprises the steps of providing a semiconductor wafer surface having residue thereon, where the residue is a consequence of a planarization process, and then contacting the residue with a cleaning composition to remove at least some of the residue from the surface. In a preferred embodiment of the invention, the cleaning composition is a solution comprising tetraalkylammonium fluoride (TAAF) of the formula (R)4NF wherein each occurrence of R is independently a C1-22alkyl group. In another preferred embodiment of the invention, the composition results from the combination of solvent, hydrofluoric acid (BF) and tetraalkylammonium hydroxide (TAAH), where TAAH has the formula (R)4NOH and each occurrence of R is independently a C1-22alkyl group.
Another aspect of the invention is a process for fabricating a semiconductor device. The process includes the steps of providing a semiconductor device having a surface, and then planarizing the surface to provide a planarized surface having residue thereon. The residue is contacted with a cleaning composition as described above. In this way, a semiconductor device may be planarized, and then residue from the planarization process may be subsequently removed.
A related aspect of the invention is a semiconductor device that has been prepared by the above-described process.
A further related aspect of the invention is a preferred process for preparing a cleaning composition useful in the above-processes. The preferred process includes the steps of adding hydrofluoric acid (HF) to water to form dilute HF, and then adding TAAH to the dilute HF. In a preferred embodiment of the invention, 0.01 to 100 volume parts of 0.1 to 50 wt % aqueous HF are added to 100 volume parts of water to form dilute HF, and then 0.01 to 100 volume parts of 0.1 to 25 wt % aqueous TAAH are added to the dilute HF. More preferably, 0.1 to 10 volume parts of 10 to 50 wt % aqueous HF are added to 100 volume parts of water to form dilute HF, and then 0.1 to 10 volume parts of 5 to 25 wt % aqueous TAAH are added to the dilute HF. Preferably, the TAAH is tetramethylammonium hydroxide. In a preferred embodiment, at least one organic solvent and/or at least one surfactant is additionally added to the cleaning composition.
The invention is also directed to a preferred composition that may be used to clean residue from a surface previously subjected to planarization. According to one aspect of the invention, the composition results from the combination of water, hydrofluoric acid (HF), tetraalkylammonium hydroxide and at least one of organic solvent and surfactant. According to another aspect of the invention, the composition results from combining tetraalkylammonium fluoride and at least one of water, organic solvent and surfactant. In a preferred embodiment, the cleaning composition results from the combination of water, 0.1-49 wt % aqueous HF and 0.1-25 wt % (aqueous or non-aqueous) TAAH in a water:HF solution:TAAH solution volume parts ratio of 100:0.0.1-100:0.01-100. More preferably, the volume parts ratio is 100:0.1-10:01.-10, and still more preferably the volume parts ratio is 100:0.5-2:0.5-5. One preferred cleaning composition has an alkaline pH, for example, a pH of about 9 to about 13, and preferably about 11. Another preferred cleaning composition has an acidic pH, for example, a pH of about 1 to about 5, preferably about 3.5. The group xe2x80x9cRxe2x80x9d in the TAAH or TAAF is preferably independently C1-4alkyl at each occurrence, where the TAAH is preferably tetramethylammonium hydroxide. A surfactant is preferably present in the composition at a concentration of about 0.001 to about 5.0 wt %, more preferably about 0.01 to 1 wt % based on the total weight of cleaning composition, where preferred surfactants are nonionic, and suitable surfactants are TRITON-X (Rohm and Haas Co., Philadelphia, Pa.) and ACATIONOX (Baxter-Scientific, McGaw Park, Ill.). An organic solvent may be present in a preferred composition, where the organic solvent is preferably present at a concentration of about 0.01 to about 95 wt % based on the total weight of the composition. In a preferred embodiment, the organic solvent contains hydroxyl functionality. Suitable organic solvents include methanol, ethanol, n-propanol, iso-propanol, ethylene glycol and propylene glycol.
These and other aspects of this invention will become apparent upon reference to the following detailed description and attached drawings.